Aldanella attleborensis (Mollusca) from Cambrian Stage 2 of the Three Gorges Area and Its Stratigraphic Implications

Simple Summary Aldanella attleborensis, is a mollusk fossil found worldwide; its first appearance datum (FAD) has been suggested as one of the potential markers for defining the base of Cambrian Stage 2. However, A. attleborensis was uncommonly recovered in the Cambrian strata of South China, and if occurring, was usually described as Aldanella yanjiaheensis or an indeterminate species in the literature due to lack of detailed taxonomic work. That is an obvious obstacle to correlate and subdivide the Cambrian strata in South China and worldwide. Recently, numerous specimens of Aldanella were collected from Member 5 of the Yanjiahe Formation in the Three Gorges area, South China, which is the type locality of A. yanjiaheensis. Morphological and taxonomic study of these specimens supports that A. yanjiaheensis is a junior synonym of A. attleborensis. Shell microstructure and muscle scar imprints on its internal mold surface indicate that A. attleborensis may belong to the gastropods. The occurrence of A. attleborensis in Member 5 of the Yanjiahe Formation suggests that this stratigraphic unit shall be of Cambrian Stage 2 in age, and can be correlated with the Dahai Member of the Zhujiaqing Formation in Yunnan, uppermost Kuanchuanpu Formation in Shaanxi, and the uppermost Tianzhushan Member of the Dengying Formation in Hubei, South China. Meanwhile, using the FAD of A. attleborensis as a bio-marker, the upper Terreneuvian strata in South China, Siberia, Avalonia, and Estonia can be well correlated. Abstract Some small shelly fossils are important index fossils for global stratigraphic subdivisions and correlations of the Cambrian Terreneuvian. The first appearance datum (FAD) of the cosmopolitan mollusk Aldanella attleborensis has been suggested as one of the potential markers for defining the base of Cambrian Stage 2. Aldanella fossils were uncommon in South China, and if occurring, were often described as Aldanella yanjiaheensis, A. attleborensis, or indeterminate species in the literature, while A. yanjiaheensis was often taken as a junior synonym of A. attleborensis. Nevertheless, a detailed taxonomic revision of A. yanjiaheensis based on material from its type locality awaits to be made. In this study, we systematically re-evaluated A. yanjiaheensis based on the numerous specimens collected from the base of Member 5 of the Yanjiahe Formation in the Three Gorges area, western Hubei Province of South China. Detailed taxonomic comparison further substantiates that A. yanjiaheensis is a junior synonym of A. attleborensis, signifying its strong potential for a global correlation across paleocontinents. Morphological parameter analyses indicate that the length and width of shell tube of A. attleborensis shows allometric growth. The nearly cosmopolitan distribution and characteristic morphology of A. attleborensis indicate that it can play a significant role in the subdivision and correlation of Cambrian Stage 2. The co-occurrence of A. attleborensis and Watsonella crosbyi from the base of Member 5 of the Yanjiahe Formation corroborates that Member 5 belongs to Cambrian Stage 2.


Introduction
The Cambrian radiation is one of the most significant bio-revolutions in Earth's history, and the sudden appearance of Small Shelly Fossils (SSFs) represents a crucial episode of this radiation event [1][2][3]. Microscopic mollusks, which are the major components of SSF assemblages, are important for studying the origin and early evolution of mollusks [4][5][6][7][8] as well as for the subdivision and correlation of pre-trilobitic Cambrian strata [4,9]. At present, the definition of the base of Cambrian Stage 2 (Terreneuvian Series) has not been determined yet. Given the cosmopolitan distribution of two molluscan taxa in Cambrian Terreneuvian strata, the first appearance datum (FAD) of Aldanella attleborensis Shaler and , internal molds, CUBar186-11; (A1), the measurement scheme of number of whorls, (A2), the measurement scheme of diameter, (A3), the measurement scheme of height; (B), internal mold, CUBar250-2. Different color definitions: green-aperture; red-mucro; pink-protoconch; yellow-last whorl; pink and purple-spire. Symbols and abbreviations are as follows: DG N -greater shell diameter at the stage of N whorls; DL N -lesser shell diameter at the stage of N whorls; DG N−0.5 -greater shell diameter at the stage of N-0.5 whorls; DG N−1 -greater diameter at the stage of N-1; H N -shell height at the stage of N whorls. HS N -spire height at the stage of N whorls; HA N -height of aperture at the stage of N whorls; WA-aperture width; WLW-width of shell tube of the last whorl; K exp -ratio of greater shell diameter to the diameter of the previous whorl; K-ratio of shell height to greater diameter of shell; K iso -ratio of greater to lesser diameters of shell.

Systematic Palaeontology
Emended diagnosis. (After [13]) Shell small, low-conical, turbospirally coiled, dextral or sinistral. Whorl surface flattened or slightly convex, and periphery rounded. Aperture oval in outline, oblique to periphery. Spire projecting onto the last whorl surface. Umbilicus varying in width, with or without opening umbilical perforation. Small, cap-shaped protoconch faintly separated from teleoconch. One or two septa occasionally developed in spire. Shell ornamented with growth lines and folds.
Remarks. The genus Aldanella was originally established by Vostokova in 1962 [23] based on material from Siberian Platform and on previous figured specimens from Massachusetts while the latter was inappropriately filed under the genus Pleurotomaria [10]. Although the morphology was very similar to that of Aldanella, Golubev [31] erected the genus Paraaldanella (type species P. kotuica) based on specimens from the western Anabar region. Parkhaev and Karlova [13] regarded the diagnostic features of P. kotuica within the intraspecific variation of Aldanella utchurica, and considered P. kotuica as a junior subjective synonym of A. utchurica. Therefore, Paraaldanella has been treated as a junior synonym of Aldanella. The low-conical shell of Aldanella is similar to that of dextral Philoxenella Vostokova, 1962 [23] and sinistral Barskovia Golubev, 1976 [31], but the difference is that both Philoxenella and Barskovia have circular apertures. From the apical view, Aldanella is similar to Pelagiella Matthew, 1895 [65], but the difference is that the latter has a flattened spire, and a larger, approximately triangular aperture.
Occurrence      Material. All the specimens (more than 100) studied herein were recovered from the base of Member 5 of the Yanjiahe Formation in the Yanjiahe and Gunziao sections. The original shells of A. attleborensis were dissolved by either diagenesis or acid maceration in the lab. Most specimens were preserved as phosphatic steinkerns exhibiting a hollow apical part and a stuffed apertural part ( Figure 2E,F3), but some specimens had partial Biology 2023, 12, 261 9 of 21 phosphatic external coatings. Because phosphate usually precipitates first from the internal surface of the shell/skeleton during diagenesis [68], internal molds are generally compactly preserved with a fine surface and occasionally with imprints of shell microstructures.   Diagnosis. Shell turbospiral, low-conical, and dextral. Umbilicus broad, upper whorl surface flattened, and periphery rounded. Aperture oblique oval. Expansion ratio of the shell moderate. Shell ornamented with growth lines. Protoconch small, cap-like with a mucro. Distinction between protoconch and teleoconch inconspicuous.
The internal molds are mostly smooth ( Remarks. Aldanella attleborensis was originally described as Pleurotomaria (Raphistoma) attleborensis by Shaler and Foerste [10] from the Cambrian strata of Massachusetts. When the genus Aldanella was established by Vostokova in 1962 [23] based on samples from the Siberian Platform, Pleurotomaria (Raphistoma) attleborensis was designated as its type species. Owing to the specific morphological features, Aldanella can be easily distinguished from other Cambrian mollusks. However, the endemic features and morphologic variations have led to an over-splitting taxonomy of Aldanella. After several rounds of revisions [13,39], A. kunda, A. rozanovi, A. costata, A. patelliforma, and A. yanjiaheensis have been regarded as junior synonyms of A. attleborensis based on the similar diagnostic features that were considered to be intraspecific variations. However, a detailed analysis of A. yanjiaheensis based on the materials from its type locality has not been performed.
In South China, specimens of Aldanella were first recovered from the Yanjiahe Formation of the Three Gorges area by Chen [33], and a new species, Aldanella yanjiaheensis, was established because these specimens were different from Aldanella ex. gr. attleborensis by having a smaller shell diameter. On account of the similarities in morphology, Landing [39] considered A. yanjiaheensis as a junior synonym of A. attleborensis. Although Aldanella has been occasionally reported in many locations in China [9,19,20,[44][45][46][47][48][49]69], the scarcity of specimens has limited paleontologists to give a detailed taxonomic revision.
In this study, 109 specimens of Aldanella collected from the 'A. yanjiaheensis' type locality were studied and measured (Supplementary Table S1). All the specimens are dextral, and low-conical, the shells diameter ranges from 1000 to 2400 µm, and the expansion ratio is moderate. The protoconch of the specimens is small, about 100-200 µm long, and always bears a mucro. The shell whorl surface is more flattened, and ornamented with traverse folds or growth lines. Except for the larger shell diameter, the morphological features and ornamentation of Aldanella specimens from the Yanjiahe Formation are consistent with A. attleborensis. So, our current study formally confirmed that the species A. yanjiaheensis Chen, 1984 [33] is a junior synonym of A. attleborensis [13,39].
Aldanella attleborensis differs from A. golubevi by the dextral shell and smaller expansion ratio ( Figure 6C); from A. sibirica by the larger (6A) and higher (6F) shell, and the more flattened upper whorl surface; from A. crassa by the smaller shell diameter (6A), smaller expansion ratio (6C), and higher whorl number of the shell (6B); from A. operosa by the absence of septum structure on the apical part, and much larger shell diameter (6A); and from A. utchurica by its much lower shell. ( Figure 6F). The shell height might be more affected by the obliquity and oblateness of shell tube cross section.
The convex polygons appear on the spire of internal mold surface, and are about 20-60 µm in diameter. The convex polygons are varying in shape, mainly circular or nearly circular, and always separated by shallow grooves [21,72,73,75]. Conversely, in some literature, the space between convex polygons is large and appears as concave polygons. Hence the convex and concave polygons seem to alternate on the spire of internal mold of Aldanella [40,74]. Similar convex polygonal impressions were described on the apex of Oelandiella and Securiconus, and were mostly interpreted as imprints of the prismatic layer [40,73,75]. The imprints of the prismatic layer always preserved as negative or positive reliefs on the molluscan internal molds [71]. Different to the prismatic layer imprints, the convex polygons of Aldanella are restricted to the apical areas [21,40,[72][73][74][75] and no tubercles in the polygons. In addition, the polygons that are interpreted as imprints of the prismatic shell layer are much smaller (10-20 µm), although the varied size of convex polygons has been attributed to the growth stages and competition for space [73]. The convex polygons of Aldanella have been interpreted as the impressions of cells of the outer epithelium [13,74], but this view remains to be investigated.
Inner layer laminar microstructure, the only preserved microstructure on the A. attleborensis of the Yanjiahe Formation, appears on the internal mold surface near the aperture ( Figures 2J2 and 3H,I). Different to the laminar microstructures of Pojetaia with a rounded edge, the lamella of herein shows two different forms: straight edge ( Figure 3H) or sharptoothed edge (Figures 2J2 and 3I). The orientation of the lamella points to the suture on the upper whorl surface and to the umbilicus on the base. This type of microstructure is common in Cambrian mollusks, such as Anabarella, Watsonella, Fordilla, Pojetaia, and Pelagiella [20,70,71,76], and is interpreted as a foliated aragonite [70].

Muscle Scars
Muscle scars provide important information to study the morphology and phylogeny of ancient univalved mollusks. Compared to most cap-shaped mollusks that have paired muscle scars on the dorsal margin, apical area, or sub-apical area [77,78], the muscle scars are always difficult to see in coiled gastropod shells where the muscle attachment site is always restricted on the umbilical area, as a columellar scar [79].
On the internal mold surface of A. attleborensis, a spiral structure is found on the umbilical area ( Figure 2H4). The spiral structure appears as about three-quarters of a whorl and vanishes near the aperture. The spiral structure is ribbon-shaped, and the width increases in the progress of growth. A similar structure has been interpreted as columellar muscle attachment on the shell umbilicus of A. rozanovi and A. kunda (junior synonyms of A. attleborensis) [26,40]. The difference is that on A. rozanovi, the spiral structure consists of honeycomb patterns with a diameter up to 6-8 µm. Isakar and Peel [40] urged caution to compare the honeycomb patterns on A. rozanovi umbilicus [26] with gastropod retractor muscles, because the honeycomb pattern is similar to the convex polygons on the spire of the internal molds. However, they ignored the fact that the convex polygons are much larger than the honeycomb patterns. In addition, on the apical surface of A. kunda, there exists an equivocal spiral ridge interpreted as the trace of a spirally migrating shell attachment muscle [40]. In Paleozoic helically coiled gastropods, ribbon-like muscle scar appeared on the umbilical area of internal molds, such as Ordovician Tropidodiscus [80], Silurian Megalomphala [81], Carboniferous Porcellia [82], and Bellerophon [81], but it is rare in Cambrian gastropods. The muscle scar on Tropidodiscus appeared on the umbilical shoulder and was preserved as a slightly depressed zone [80]. The position and the shape of Tropidodiscus muscle scar is consistent with that of Aldanella. The muscle scar of Megalomphala was preserved as smooth area on the umbilical wall of the internal mold, separated by two spiral ridges from the internal mold [81]. On the Porcellia, a single ribbonlike muscle scar is longer and narrower, and lies on the basal surface of the whorl [82]. The muscle scar of Bellerophon is located on the umbilical shoulder, but the difference is that Bellerophon has a pair of muscle scars and the scars were preserved as a raised zone [81].
Besides Aldanella, similar ribbon-like structures also have been described on Cambrian coiled mollusk Pelagiella [63,83,84]. That is an important characteristic to support these genera as gastropods [26,27]. Comparing the ribbon-like structure position and shape of A. attleborensis with that of the muscle scars on the upper Paleozoic gastropods, as well as the columellar muscles on living spirally coiled gastropods, it is herein considered that the spiral structure on the umbilicus of Aldanella is the shell muscle attachment area, although some of upper Paleozoic gastropods have a pair of muscle scars.

Systematic Position of Aldanella
The systematic position of the genus Aldanella is controversial, and has been suggested to gastropods [13,[24][25][26][27], helcionelloids [40,85], hyoliths [66], or polychaetes [86,87]. The phosphatic preservation of the shell and the absence of the soft body provide extremely limited evidence to study the phylogeny and affinity of this genus. The imprints of the shell microstructures and the shell muscle attachments have been recognized as significant information to reveal the systematic position of early mollusks [20,70,71,88,89], and therefore, it is necessary to research these features on Aldanella.
The presence of laminar microstructure on the internal molds of A. attleborensis provides crucial information to analyze its systematic position. As the laminar microstructure reported in Pelagiella has been suggested to be nacreous by Thomas et al. [27], the presumption that Pelagielliformes microstructure is comparable to that of hyoliths in lacking nacre is inappropriate. Most importantly, the operculum, preserved in hyoliths, has never been found in Pelagielliformes. Apparently, Aldanella is not a hyolith. The septa presented in the initial shell of A. operosa exclude the possibility that Aldanella is a polychaete [26]. Furthermore, laminar microstructure was never discovered in polychaetes [90]. In addition, Aldanella was suggested as helcionelloids because they share an endogastric shell. However, the muscle scar preserved on the umbilicus area of A. attleborensis is more similar to that of Paleozoic gastropods than the paired muscle scars of helcionelloids. Hence, according to the muscle scars similarity of Aldanella and upper Paleozoic gastropods in position and shape, the appearance of laminar microstructure on the Aldanella shell, the presence of protoconch and regular septa in the initial part of the shell, as well as the asymmetrically turbospiral shell, Aldanella is more likely to be a gastropod than helcionelloids, hyoliths, or polychaetes.

Ontogeny of Aldanella attleborensis
Ontogenetic analysis is a useful method for studying the origin and early evolution of gastropods. Based on the fossil materials of Paleozoic, many achievements on gastropod ontogeny have been made [91,92]. However, the ontogeny of early Cambrian gastropods, such as Aldanella and Pelagiella, is rarely systematically studied due to limited specimens. The shell of Aldanella was ontogenetically identified in two (protoconch and teleoconch) or three (protoconch I, protoconch II, and teleoconch) developmental stages [93].
The morphology of A. attleborensis analyzed in this study can be compared with Siberian materials, and the protoconch and teleoconch of the shell is faintly separated. The protoconch of A. attleborensis is small, approximately 100-200 µm in length. The small size of the protoconch may indicate the existence of the planktonic larval stage during development [24,91,93,94], which is consistent with its worldwide distribution in the lower Cambrian strata. The protoconch always bears a mucro on those specimens preserved with external coating, but the protoconch of the internal mold is smoothly rounded (Figure 4A2,H2). This is a unique character that has been found on the external coating of Aldanella [66,72]. The size and shape of the mucro is variable, and it looks like a short column ( Figure 4C2,E2,F2) or blunt cusp ( Figure 4A2,B2,D2,G2). As shown in the Micro-CT images ( Figure 4G4-G6), when the original shell is dissolved, the mucro is expressed as a hollow space. This means that the mucro is an original solid protrusion on the protoconch. Moreover, the mucro has no surface ornamentation. Although the function of the mucro remains unknown, there is no ground to conclude that it may represent a discrete stage of larval shell development, as suggested by Dzik [95] and Thomas et al. [27].
Most of the specimens studied herein were partially broken, especially at the protoconch and aperture. Nevertheless, the well-preserved inner lip of the aperture on some specimens with phosphatic coating (Figures 2G2 and 3A) indicates that the steinkerns almost represent complete specimens, not just broken off pieces of larger shells [66]. The specimens of A. attleborensis described herein range from 1067 µm to 2397 µm in diameter ( Figure 6A), and from 1.50 to 2.55 in the number of whorls ( Figure 6B). The wide range of sizes and whorl numbers of the shells suggests that both juvenile and adult stages are preserved. In addition, the box-plot of shell diameter and whorl numbers showed that the adult shell size mainly ranges from 1400 µm to 1900 µm, and the whorl numbers correspond to approximately 2. The strong linear regressions of the expansion and isometry rates ( Figure 6C,D) indicate that the shell tube width (the straight-line distance from suture to periphery) of A. attleborensis constantly increases. Owing to the turbospirally coiled shell, the length of shell tube (the helical-line distance from protoconch to aperture) is difficult to measure. Compared with the diameter of the shell, the number of whorls could be more accurate in estimating the longitudinal growth rate of the shell tube. Although the general trend is positive, the weak linear regression of the shell diameter and number of whorls ( Figure 6E) suggests that the final shell size is not strongly correlated with the whorl number, and the length of shell tube of the whorl is allometric growth relative to its width. In addition, the shell height had a weak relationship with the number of whorls ( Figure 6F). The shell height might be more affected by the obliquity and oblateness of shell tube cross section.

Stratigraphic Implications of Aldanella attleborensis
With the progress of Cambrian stratigraphy subdivision, more than half of the Global Boundary Stratotype Section and Points (GSSPs) of Cambrian stages have been ratified. However, due to high levels of endemism and the facies-restricted distribution of fossils, as well as the absence of the pelagic skeletonized metazoans used for global correlation, formal ratification of Stage 2, Stage 3, and Stage 4 is difficult [41,96]. Nevertheless, SSFs play an important biostratigraphic role for the subdivision and correlation of pre-trilobitic (Terreneuvian) Cambrian strata [4,9,[15][16][17][18]. For example, the widely distributed A. attleborensis or W. crosbyi both have been suggested as a potential marker for defining the base of Cambrian Stage 2.
In this study, numerous specimens of A. attleborensis, co-occurring with W. crosbyi, have been discovered from Member 5 of the Yanjiahe Formation of the Three Gorges area, South China. Besides the Yanjiahe Formation, A. attleborensis was also discovered from the uppermost Huangshandong Member (Tianzhushan Member) of the Dengying Formation in Tianzhushan of the Three Gorges area [47]. The thickness of the Huangshandong Member is extremely varied at different sections in the Three Gorges area (0.1-8 m) [43,44,97], and the SSF assemblages are often mixed [43,44], or SSF zone 1 and 2 could be recognized [46,97]. However, besides A. attleborensis, W. crosbyi was also found in the uppermost Tianzhushan Member from the Tianzhushan Section [97], Huangshandong Section [43,97], and Songlinpo Section (unpublished data). This may indicate that the SSF zone 3 occasionally exists in the uppermost Tianzhushan Member at some sections with a relatively thick unmixed sequence of this lithological unit.
The similar phenomenon also appears in southern Shaanxi. The Kuanchuanpu Formation is well-exposed in southern Shaanxi and contains abundant SSFs. Nevertheless, scarce materials of A. attleborensis and W. crosbyi were only recovered in the uppermost Kuanchuanpu Formation at the Yuanjiaping Section of Ningqiang County [43]. This indicates that the sediments containing SSF zone 3 are rare in the Kuanchuanpu Formation of southern Shaanxi. In addition, Aldanella attleborensis was discovered from the Dahai Member of the Zhujiaqing Formation in Eastern Yunnan [9,42], but its first occurrence horizon is much higher than that of W. crosbyi [42].
Using A. attleborensis as a biostratigraphic marker, Member 5 of the Yanjiahe Formation, uppermost Tianzhushan Member, uppermost Kuanchuanpu Formation, and the Dahai Member can be well correlated. Therefore, the FAD of A. attleborensis has a potential to correlate the pre-trilobitic strata in South China. In addition, A. attleborensis was documented in the upper part of the Yurtus Formation, Aksu-Wushi region, Tarim [48,49,69], but the age of the uppermost Yurtus Formation has been suggested to be Qiongzhusian equivocally [49]. Therefore, in Tarim the detailed stratigraphic correlation still needs to be further confirmed. With the exception of South China, Aldanella attleborensis is also widely distributed in the pre-trilobitic strata of most paleocontinents, such as Avalonia, Baltica, Siberia, etc. (Figure 7). The materials of A. attleborensis in Siberia are extremely rich, and its stratigraphic range were suggested to be the whole Tommotian [13]. However, A. attleborensis may appear much lower than W. crosbyi in Siberia [21,50] and the hiatus between the Pestrotsvet Formation and the underlying Ust-Yudoma Formation [101] makes it difficult to evaluate the accuracy of the FAD [12]. The FADs of W. crosbyi and A. attleborensis in Avalonia correspond to the base of the Tommotian, but the temporal range of W. crosbyi and A. attleborensis is much longer and their LADs (last appearance data) might not be limited to the Tommotian [41]. In western Mongolia, the original description of W. crosbyi assemblage zone suggested a Tommotian age [100], but it was revised to be pre-Tommotian in age by Smith et al. [102] based on chemostratigraphic data. Aldanella in western Mongolia was uncommon, appeared much higher than W. crosbyi, and only one specimen, assigned to Aldanella sp., was reported from the probable Botoman Stage [100]. Although this indeterminate species has been revised to A. attleborensis [13], the horizon and taxonomy of A. attleborensis in western Mongolia [100] is questionable [21]. Aldanella attleborensis has been reported in the Kestla Member of the Lontova Formation in northern Estonia [40], and its age is limited to the Tommotian [67,99].
In summary, A. attleborensis is one of the most widely distributed SSFs and is an important fossil for both regional and global correlation of Cambrian Terreneuvian ( Figure  8). Based on its wide geographic distribution and distinct turbospirally coiled shell, A. attleborensis has the potential to be an index fossil for defining the base of Cambrian Stage  [98] and [20]; the distribution data from [9,13,21,33,39,40,49,99,100] and herein.
With the exception of South China, Aldanella attleborensis is also widely distributed in the pre-trilobitic strata of most paleocontinents, such as Avalonia, Baltica, Siberia, etc. (Figure 7). The materials of A. attleborensis in Siberia are extremely rich, and its stratigraphic range were suggested to be the whole Tommotian [13]. However, A. attleborensis may appear much lower than W. crosbyi in Siberia [21,50] and the hiatus between the Pestrotsvet Formation and the underlying Ust-Yudoma Formation [101] makes it difficult to evaluate the accuracy of the FAD [12]. The FADs of W. crosbyi and A. attleborensis in Avalonia correspond to the base of the Tommotian, but the temporal range of W. crosbyi and A. attleborensis is much longer and their LADs (last appearance data) might not be limited to the Tommotian [41]. In western Mongolia, the original description of W. crosbyi assemblage zone suggested a Tommotian age [100], but it was revised to be pre-Tommotian in age by Smith et al. [102] based on chemostratigraphic data. Aldanella in western Mongolia was uncommon, appeared much higher than W. crosbyi, and only one specimen, assigned to Aldanella sp., was reported from the probable Botoman Stage [100]. Although this indeterminate species has been revised to A. attleborensis [13], the horizon and taxonomy of A. attleborensis in western Mongolia [100] is questionable [21]. Aldanella attleborensis has been reported in the Kestla Member of the Lontova Formation in northern Estonia [40], and its age is limited to the Tommotian [67,99].
In summary, A. attleborensis is one of the most widely distributed SSFs and is an important fossil for both regional and global correlation of Cambrian Terreneuvian (Figure 8). Based on its wide geographic distribution and distinct turbospirally coiled shell, A. attleborensis has the potential to be an index fossil for defining the base of Cambrian Stage 2 [13,14,67]. In addition to A. attleborensis, another mollusk, W. crosbyi, which is broadly similar in stratigraphic distribution to A. attleborensis, such as South China, Siberia, Avalonia, Australia, Armorican France, and Mongolia (see [12,20] and references cited therein), has also been suggested as a potential marker for defining the base of Cambrian Stage 2 [12,103].

Conclusions
Numerous specimens of Aldanella were collected from Member 5 of the Yanjiahe Formation at the Yanjiahe and Gunziao sections in the Three Gorges area, South China. This study shows that A. yanjiaheensis is morphologically consistent with A. attleborensis in many aspects, such as the small turbospiral shell, moderate expansion rate, up to 2.5 tightly coiled whorls, oblique oval aperture, and growth lines occurring on the shell. Our work formally confirms that the species A. yanjiaheensis Chen, 1984 is a junior synonym of A. attleborensis.
Two developmental stages, protoconch and teleoconch, of the shell of A. attleborensis were identified. The small protoconch of the shell bears a distinct mucro. The strong linear regressions of the expansion and isometry rates indicated that the shell tube width constantly increases. However, the weak linear regression of the shell diameter and number of whorls suggests that the shell tube of the whorl is allometric growth. The spiral muscle scar structure further confirms that Aldanella is more likely to be a gastropod.
Aldanella attleborensis widely occurred in Cambrian paleocontinents (South China, Mongolia, Siberian, Baltica, Avalonia, etc.) from low to high paleolatitudes. The stratigraphic range of A. attleborensis indicates that it was mainly restricted to Cambrian Stage 2. Aldanella attleborensis can be easily distinguished by its characteristic features from other congeneric species. Therefore, similar to W. crosbyi, the FAD of A. attleborensis could also be a potential candidate marker for defining the base of Cambrian Stage 2. The co-occurrence of A. attleborensis and W. crosbyi at the base of Member 5 of the Yanjiahe Formation indicates that Member 5 belongs to Cambrian Stage 2.